1. Field of the Invention
The present invention relates to a metal gasket that can be suitably used for automobile engines.
2. Description of the Related Art
A metal gasket widely employed for automobile engines is one which is composed of a single or a plurality of gasket component sheet(s). The gasket component sheet has an opening formed to face a combustion chamber, a bead formed to surround the opening, and a stopper formed on the inner circumference side of the bead to prevent the flattening of the bead.
An example of widely used stoppers is formed by folding back the peripheral edge of a gasket component sheet opening so that this folded part is used as a stopper. Meanwhile, it has recently been proposed, as disclosed in Japanese Patent Kokai Publication No. 2004-144119, to form a stopper with a wave-shaped cross section, by means of press molding, on the inner side of a bead on a gasket component sheet and concentrically with the opening. This Japanese patent publication also discloses a stopper which is formed by pressing a part of a gasket component sheet on the inner circumference side of a bead into a sine wave shape, then smoothing the top and bottom of each wave so that the slopes of the wave become thicker than the top and bottom.
Japanese Patent Kohyo Publication No. 2004-503731 discloses a metal gasket in which a stopper with a sine wave shape is formed by press molding on the inner side of a bead on a gasket component sheet so that the slopes of each wave become thinner than the top and the bottom of the wave.
Formation of a stopper only on the inner circumference side of a bead will lead to a problem of poor balance of contact pressure around a combustion chamber. Therefore, it has also been proposed to form a stopper both on the inner and outer circumference sides of a bead to make the contact pressure balance uniform (see Japanese Patent Kokai Publication No. H11-2324, for example).
It is known that stress amplitude occurs in a metal gasket bead, repeating compression and decompression, due to pressure fluctuation in cylinders of an engine during engine operation, and such stress amplitude constitutes a factor that causes cracks in the bead. When a plurality of waves with a narrow width (with a short wavelength) are arranged to function as a stopper as described in the Japanese Patent Kokai Publication No. 2004-144119 and Kohyo Publication No. 2004-503731, in particular, the ratio of height to width of each wave is so large that fatigue fracture will possibly occur in the stopper if the waves are formed by a same method as the bead. More specifically, according to conventional techniques, a bead is formed using a mold having a molding surface substantially conforming to the final shape of the bead, and this fact raises a problem that a large frictional force is generated between the top of a male punch and a blank metal sheet. The frictional resistance thus generated restricts the lateral extension of the part of the blank metal sheet contacted with the top of the punch. When waves are formed into a required shape, only the opposite slopes of the waves are extended but the top and bottom thereof are not extended significantly. As the result, the thickness at the top and bottom of each wave becomes larger than the thickness of the slopes. This tendency becomes more severe as the ratio of height to width of a bead becomes larger. Therefore, if waves are formed with a large ratio of height to width as described in Japanese Patent Kokai Publication No. 2004-144119 and Kohyo Publication No. 2004-503731, there is a risk that fatigue fracture occurs at the top of the waves in the stopper.
In addition, although Japanese Patent Kokai Publication No. 2004-144119 discloses a stopper having waves the top of which is thin, these waves are formed by press molding a blank metal sheet into a sine wave form and then smoothing the top of each wave. The resistance to compression of the stopper may be enhanced by making the slopes of the wave thicker in this manner. However, cracks are liable to occur at the tops of waves which are subject to maximum stress amplitude. Moreover, it is required to smooth the tops of waves, after the press molding process, with the opening edge fixed to prevent the wave width being enlarged. This makes the manufacturing process complicated and requires a special mold for the smoothing process.
Whereas, according to Japanese Patent Kokai Publication No. H11-2324, a stopper is formed into a thin ring shape from a rigid coating material or sintered material, on the inner and outer circumference sides of a bead, by means of coating, flame spray coating, screen printing, stencil spraying or the like. However, the thermal resistance requirement is particularly stringent to the stopper on the inner circumference side, that is arranged on the side of combustion chamber, and such stopper is subjected to load exerted in the shearing direction due to difference in coefficient of thermal expansion between a cylinder head and a cylinder block. Therefore, there is a risk that the sealing performance is deteriorated as the result of the stopper coming off.